OP go here and start to play with the numbers:
http://tvcalculator.com/
To achieve similar 27" 1080p pixel density if you upgraded to 4K you're going to need a 55" screen.
A compromise might be 2560x1440 via 32" screen which is same pixel density as 24" 1080p.
I'm not looking for "the same density" but "at least that density" so i definately don't need a 55", i need Something no larger than a 55", of course better density if always better, i was simply giving a cut off because i don't want to "downgrade" in density, upgrading it always fine.
For now the best bet seems to be those 4K 40" mentioned earlier, so far no one seems to have trashed them so i'm not sure if i it's no one using them or if it means there's nothing wrong but now that i know that they exist i'm clearly not considering anything smaller unless there's a major issue with them for my use case.